The present invention relates to trigger control for an electric power tool.
The speed of electric power tools is often controlled by means of a pull-trigger, which is used to switch on and off the motor as well as to adjust the motor speed/torque. The pull-trigger invariably incorporates an electrical switch for control, which is either a mechanical switch or a solid-state switch.
The invention seeks to provide an improved trigger controller.
According to the invention, there is provided a trigger controller for an electric power tool which is powered by a power source and includes an electric motor and a pull-trigger movable along a path for controlling the motor. The pull-trigger has a foremost home position and a predetermined full travelling distance from the home position. The controller comprising:
a mechanical switching device and a solid-state switching device connected in series between the motor and the power source, the mechanical switching device including a first moving contact mechanically associated with the pull-trigger for movement thereby,
control unit including an integrated circuit connected to the solid-state switching device for generating an adjustable control signal to turn on and off the solid-state switching device for delivering an electric current from the power source via the mechanical switching device to the motor, the current having an adjustable rms value, and
an output selector including a plurality of resistors interconnected in series and a second moving contact connectable selectively to junctions of the resistors and connected to the control unit for adjusting the control signal to operate the solid-state switching device, the second moving contact being mechanically associated with the pull-trigger for movement thereby,
the first and second moving contacts being movable by the pull-trigger to operate the mechanical and solid-state switching devices respectively at different first and second predetermined travelling positions of the pull-trigger from the home position, such that the two switching devices are caused to be initially closed at different times.
In a first embodiment, the second travelling position is downstream of the first travelling position such that the solid-state switching device is initiaily closed after the mechanical switching device.
In a second embodiment, the second travelling position is upstream of the first travelling position such that the solid-state switching device is caused to be initially closed before the mechanical switching device.
Preferably, the first and second travelling positions are different from each other by a distance substantially in the range from 1.1% to 8.1% of the full travelling distance along the path.
More preferably, the first and second travelling positions are different from each other by a distance substantially in the range from 0.1 mm to 0.5 mm of the full travelling distance along the path.
It is preferred that the first travelling position is substantially in the range from 16.7% to 61.3% of the full travelling distance from the home position along the path.
It is further preferred that the first travelling position is substantially in the range of 1.5 mm to 3.8 mm from the home position along the path.
Preferably, the first travelling position is substantially in the range from 16.7% to 61.3% of the full travelling distance from the home position along the path.
Preferably, the second moving contact is mounted on the pull-trigger for movement thereby.
It is preferred that the trigger controller includes a brake switch connected in parallel with the motor for regenerative braking, the brake switch including a moving contact mechanically associated with the pull-trigger for movement thereby.
It is further preferred that the moving contact is movable by the pull-trigger to open the brake switch at a predetermined position of the pull-trigger between the home position and the first travelling position.
It is preferred that the trigger controller includes a bypass switch connected in parallel with the mechanical and solid-state switching devices for providing a direct path from the power source to the motor, the bypass switch including a moving contact mechanically associated with the pull-trigger for movement thereby.
It is further preferred that the moving contact is movable by the pull-trigger to close the bypass switch at a predetermined position of the pull-trigger adjacent a rearmost end position thereof along the path.
More preferably, the predetermined position is substantially in the range from 5.5 mm to 7.0 mm measured from the home position.
It is preferred that the moving contact is movable by the pull-trigger to close the bypass switch after the solid-state switching device has been turned continuously on by the control unit.
It is preferred that the bypass switch and the mechanical switching device share a common moving contact that is movable by the pull-trigger to close the bypass switch after the solid-state switching device has been turned continuously on by the control unit and then to open the mechanical switching device.
The invention also provides an electric power tool which includes an electric motor and a pull-trigger movable along a path, having a foremost home position and a predetermined full travelling distance from the home position, for operating the motor, and includes, the aforesaid trigger controller.
Preferably, the electric power tool is an electric hand drill.
The invention further provides an electric power tool which includes an electric motor and a pull-trigger movable along a path, having a foremost home position and a predetermined full travelling distance from the home position, for operating the motor, and includes the aforesaid trigger controller, wherein the full travelling distance of the pull-trigger is substantially in the range from 6.2 mm to 9.0 mm.